According to conventional practices in powder metallurgy, powdered metals can be converted into a metal article having virtually any desired shape. First, the metal powder is compressed in a die to form a "green" compact having the general shape of the die. The compact is then sintered at an elevated temperature to fuse the individual metal particles together into a sintered metal part having a useful amount of strength and yet still retaining the general shape of the die in which the compact was made. The metal powders utilized can be pure metals, alloys, or a blend of these. Generally, sintering will yield a part having between about 60% and 95% of theoretical density. If particularly high density (low porosity) is desired, a process such as hot isostatic pressing will be utilized instead of sintering. However, several applications have developed for sintered parts where porosity is desired and beneficial.
A commercially significant blend of powdered metals is a blend of about 10% tin powder and about 90% copper powder which produces a sintered "bronze alloy". According to one common practice, the sintering conditions for this bronze alloy are controlled so that a predetermined degree of porosity remains in the sintered part. Such parts can then be impregnated with oil under pressure to form a so-called permanently lubricated part. These parts have found wide application as bearings and motor components in consumer products and eliminate the need for periodic lubrication of these parts during the useful life of the product.
Solid lubricants such as graphite, lead, lead alloy, molybdenum disulfide and tungsten disulfide, as well as other additives, have been incorporated in the blends for making such sintered alloys. However, the metal powders utilized have typically been commercially pure grades of copper powder and tin powder.
It has been suggested in U.S. Pat. No. 4,274,874 that a small proportion of phosphorus (from 0.2 to 1.7%) can be incorporated in a sintered bronze alloy for the purpose of improving the lubrication properties of a bearing produced from the alloy under certain extreme service conditions. The phosphorus was incorporated in the alloy by adding a predetermined proportion of a copper-phosphorus alloy to the metal powder blend before sintering. Said patent further makes reference to a Japanese patent application No. 451/60 dated Jan. 26, 1960 which discloses the use of a sintered alloy for a sliding plate for a collector for an electric car. This sintered part consists of from 0.1 to 5 weight percent phosphorus, from 5 to 18 weight percent tin, from 2 to 10 weight percent graphite, and the balance copper. The addition of phosphorus there is solely to improve the strength and hardness of the sliding plate and the sintered alloy produced could only accept an oil content of about 1%.
These, and indeed all known commercially available sintered bronze alloys undergo a small but significant change in dimensions during sintering to make porous parts. This dimensional change is typically an increase in size and is very dependent on the sintering time and temperature. The above suggested addition of a copper-phosphorous alloy to the metal powder blend before sintering has been shown to reduce the overall growth of the part during sintering. However, blends with or without the addition of a copper-phosphorous alloy are quite sensitive to the sintering conditions. That is, a small change in sintering time and/or sintering temperature will produce a significant change in the dimensions of the sintered part. Studies of this behavior show that compacts expand during the early stages of sintering, but then begin to contract. The result, for sintering conditions commonly used for producing porous sintered bronze parts, is generally a net expansion of the part. However, because of this behavior the sensitivity of a specific blend to sintering conditions may be more significant than the net expansion of the part particularly when tolerances must be maintained in a commercial product.
Therefore, it is obviously beneficial to maintain a minimum dimensional change during sintering, but it is equally important, and in some instances more important, to have dimensional change relatively insensitive to sintering conditions.